Tongue-Eating Louse
The only known parasite that destroys an organ and then becomes it
No photo available for Tongue-Eating Louse
Superpower
Cymothoa exigua enters a fish through its gills as a juvenile, crawls to the tongue, severs its blood supply until the tongue withers and falls off, then physically attaches itself to the tongue stub. The fish uses the louse as a functional replacement tongue for the rest of its life — chewing, manipulating food, and swallowing with a living crustacean. The louse feeds on the fish's blood and mucus the entire time.
Overview
Tongue-eating lice are parasitic isopods — relatives of woodlice — found in marine fish worldwide. Juveniles enter as males; if a female is already present, some remain male and mate with her. If the tongue is unoccupied, one transforms into a female and attaches permanently. The host fish is not killed — it lives a normal life span, feeding and reproducing, just with a 3cm isopod where its tongue used to be. This is the only known case in biology of a parasite functionally replacing a host organ.
Found in
Marine fish worldwide. Best documented in the spotted rose snapper off the California coast and in the Gulf of California, but infects dozens of fish species in the Atlantic, Pacific, and Mediterranean.
Things worth knowing
- 1
All tongue-eating lice are born male. The sex change is triggered by the environment — specifically by whether a female is already present in the fish's mouth.
- 2
A second male that arrives at an already-infected fish doesn't attach to the tongue — instead, he injects himself into the female's body to become her permanent internal sperm donor.
- 3
The louse is so well integrated that it moves when the fish chews, functioning as a seamless mechanical replacement. Fish with the louse appear to feed normally, though slightly less efficiently.
- 4
Finding one of these in a fish at a market is considered alarming by consumers but is completely harmless to eat — the louse is just a crustacean, like a shrimp.
- 5
The full lifecycle wasn't understood until the 1970s, and the sex-change mechanism wasn't confirmed until the 1990s, despite the species being known to science since 1769.